H2—O2(air) fuel cells are well known in the art and have been proposed as a power source for many applications. There are several types of H2—O2 fuel cells including acid-type, alkaline-type, molten-carbonate-type, and solid-oxide-type. So called PEM (proton exchange membrane) fuel cells (a.k.a. SPE (solid polymer electrolyte) fuel cells) are of the acid-type, potentially have high power and low weight, and accordingly are desirable for mobile applications (e.g., electric vehicles). PEM fuel cells are well known in the art, and include a “membrane electrode assembly” (a.k.a. MEA) comprising a thin, proton transmissive, solid polymer membrane-electrolyte having an anode on one of its faces and a cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts. A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack.
In PEM fuel cells hydrogen is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can either be in a pure form (i.e., O2), or air (i.e., O2 mixed with N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprise finely divided catalytic particles (often supported on carbon particles) admixed with proton conductive resin.
During the conversion of the anode reactant and cathode reactant to electrical energy, the fuel cell, regardless of the type, produces anode and cathode effluents that are exhausted from the fuel cell stack. Because the anode reactant (hydrogen) is supplied in a surplus quantity (i.e., above the required stoichiometric amount) the anode effluent contains unused hydrogen that represents an unused source of energy. Similarly, the cathode reactant (oxygen) is also supplied in an excess amount and as a result the cathode effluent contains excess oxygen or air that was not consumed during the production of electricity in the fuel cell stack. The amounts of hydrogen and oxygen remaining in the anode and cathode effluents is dependent upon a number of factors and will vary. For example, the efficiency of the fuel cell can impact the amount of hydrogen and oxygen that are exhausted in the respective anode and cathode effluents. Additionally, the stoichiometric operation of the fuel cell stack (i.e., the amounts of hydrogen and oxygen that are included in the respective anode and cathode reactants) will also effect the amount of remaining hydrogen and oxygen in the respective anode and cathode effluents. The anode effluent exhausted by the fuel cell stack can be either continuous or intermittent, depending upon the desired operation of the fuel cell system within which the fuel cell stack is employed.
Because excess hydrogen is exhausted from the fuel cell stack in the anode effluent, it is beneficial to capture the energy contained in the excess hydrogen. One manner of utilizing the excess hydrogen is to recycle at least a portion of the anode effluent back through the anode side of this or another fuel cell stack. To accomplish this, prior art fuel cell systems use a pump to recirculate a portion of the anode effluent back through the anode side of the fuel cell stack. However, the pumps used for such purposes are expensive and add to the cost of a fuel cell system. Additionally, the pumps are relatively large and increase the required packaging space of the fuel cell system. Therefore, it would be desirable to be able to recirculate at least a portion of the anode effluent through a fuel cell stack in a simple and low cost manner. Furthermore, it is desirable to recycle at least a portion of the anode effluent with hardware that adds relatively little or no additional required packing space to a fuel cell system.